The right position of the tongue while singing is very important. If you sing with a flat or retracted tongue you create a lot of tension, push the larynx down and you will loose resonance because the back of the throat is filled with tongue.
Read David L. Jones’ article about this subject. It is a very useful article.
The tongue spans a large area in your mouth and throat. It attaches to the hyoid bone that is situated above the larynx. You use your tongue for chewing, swallowing, tasting and speech.
Alexander massey made two videos with exercises that may look silly at first, but they are very helpful!
I often use “NG” sound to place the tongue in the right position:
Say the word “MING” and let the “NG” resonate, like “MINGNGNGNGNG”.
Now sing a song and replace the text for a humming “NG”.
It is important that you do not try to make it sound nice! And do not push the sound! The only important thing is that it FEELS effortless and smooth! You may get the feeling that you sound like a mouse…that’s okay! But is has to feel effortless.
If you add an inner smile it will feel even better! You will not only put the tongue in the right position, you will also train the edges of the vocal folds (thin vocal folds) and learn how to manage your breath support.